Antimicrobials · Carbapenems
The facts most likely to be tested
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Imipenem is a broad-spectrum carbapenem antibiotic that provides excellent coverage against Gram-positive cocci, Gram-negative rods, and anaerobes.
Cilastatin is a dehydropeptidase-I inhibitor added to imipenem to prevent renal tubular metabolism and increase urinary drug concentrations.
Imipenem carries the highest risk among carbapenems for inducing seizures, particularly in patients with renal insufficiency or a history of epilepsy.
Carbapenems are the drugs of choice for treating Extended-Spectrum Beta-Lactamase (ESBL) producing organisms.
Imipenem lacks activity against MRSA, Enterococcus faecium, and Stenotrophomonas maltophilia.
Patients with a severe IgE-mediated allergy to penicillins may exhibit cross-reactivity with carbapenems and should be monitored closely.
Imipenem-cilastatin is frequently utilized as empiric therapy for hospital-acquired pneumonia or complicated intra-abdominal infections.
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A 68-year-old male is hospitalized for a complicated diverticulitis with a suspected abscess. He has a history of chronic kidney disease and a remote history of a seizure disorder controlled with levetiracetam. He is started on broad-spectrum intravenous antibiotics. On the third day of therapy, the patient experiences a generalized tonic-clonic seizure despite therapeutic levetiracetam levels. His renal function has remained stable, but his urinary output has decreased slightly.
Which of the following medications is the most likely cause of the patient's new-onset seizure?
Imipenem-cilastatin
The patient's seizure is a classic adverse effect of imipenem, which lowers the seizure threshold, especially in patients with pre-existing neurological conditions or renal impairment.
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High yield triage
Classification
Carbapenem antibiotic combined with a dehydropeptidase-I inhibitor.
Indications
Broad-spectrum coverage for severe nosocomial infections, polymicrobial sepsis, and intra-abdominal infections.
Mechanism of Action
Binds penicillin-binding proteins (PBPs) to inhibit bacterial cell wall synthesis.
Side Effects
Seizures, nausea, vomiting, and diarrhea.
Contraindications / Monitoring
History of anaphylaxis to beta-lactams. Monitor renal function and neurological status.
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Mechanism of Action
Imipenem binds to penicillin-binding proteins (PBPs), specifically PBP-1 and PBP-2, disrupting peptidoglycan cross-linking. This leads to bactericidal cell lysis. Cilastatin is added to inhibit renal dehydropeptidase-I, preventing the rapid degradation of imipenem in the renal tubules.
Unique Properties
Imipenem is the broadest-spectrum beta-lactam, covering Gram-positive cocci, Gram-negative rods, and anaerobes. Unlike other carbapenems, it carries the highest risk of lowering the seizure threshold.
Indications
Indicated for empiric therapy in critically ill patients with hospital-acquired pneumonia, complicated intra-abdominal infections, and febrile neutropenia. It is a potent agent against Pseudomonas aeruginosa and Acinetobacter species.
Pharmacokinetics
Imipenem is not orally absorbed and must be administered intravenously. It is primarily excreted via the kidneys; dosage adjustments are mandatory in patients with renal impairment to prevent drug accumulation.
Side Effects & Adverse Events
Common side effects include nausea and vomiting. The most feared adverse effect is seizures, particularly in patients with pre-existing CNS disorders or renal failure. Anaphylaxis is a risk in patients with cross-reactivity to other beta-lactams.
Contraindications
Absolute contraindication includes a history of anaphylaxis or severe hypersensitivity to any beta-lactam antibiotic. Use with extreme caution in patients with a history of epilepsy or seizure disorders due to the risk of drug-induced neurotoxicity.
Monitoring
Monitor serum creatinine and creatinine clearance to adjust dosing. Observe patients for neurological changes, including tremors or altered mental status, which may precede a seizure.
Clinical Pearls
Always remember the Cilastatin component is purely to prevent renal metabolism, not to enhance antibacterial activity. If a board question describes a patient with renal failure receiving imipenem who develops myoclonus or seizures, the drug is the culprit.